GIFT  OF 


Ftb  15     ISit 

DATA  ON  ARTIFICIAL  LIGHTING 


Supplementary  to  Section  II  of ff Scientific 
Office  Management"  by  W.  H.  Leffingwell 


A.  W.  SHAW  COMPANY 

CHICAGO      NEW  YORK 
LONDON 


COPYRIGHT,  1917,  BY 
A.  W.  SHAW  COMPANY 

PBINTED  IN  U.  B.  A. 


DATA   ON   ARTIFICIAL  LIGHTING 


Where  gas  light  is  used  incandescent  inverted  mantles  wil 
usually  give  the  best  results.  In  electric  lighting  rapid  prog- 
ress within  recent  years  has  made  possible  a  wide  range  of 
choice  in  the  kind  of  lighting,  and  there  is  naturally  more  or  less 
dispute  as  to  which  light  is  really  the  most  efficient. 

In  considering  the  efficacy  of  lighting  methods  it  is  important 
to  know  in  just  what  respect  one  method  is  more  efficient  than 
another.  It  is  possible  that  while  one  method  may  consume  less 
current  it  will  give  light  of  much  poorer  quality  or  with  a 
harmful  effect  upon  the  eyes  of  the  workers.  It  is  therefore 
necessary  for  us  to  know  which  is  more  important,  economy  in 
the  consumption  of  current  or  economy  in  labor. 

There  are  perhaps  three  methods  of  lighting  in  common  use. 
In  the  direct  lighting  method  the  source  of  light  is  visible  and 
the  light  rays  shine  directly  upon  the  work.  In  the  semidirect 
or  semiindirect  lighting  systems  the  source  of  light  is  visible, 
but  only  a  small  portion  of  the  light  rays  shines  directly  upon 
the  work,  and  these  rays  are  still  further  modified  by  translucent 
globes.  In  the  indirect  lighting  systems  the  source  of  light  is 
invisible,  all  the  rays  being  thrown  against  the  ceiling  or  some 
other  light  colored  surface  and  reflected  down  upon  the  work. 

Direct  lighting  is  the  oldest  and  most  common  method,  and 
so  far  as  mere  consumption  of  current  is  concerned,  much  the 
cheapest  of  the  three.  Nowadays,  however,  the  actual  economy 
of  this  method  is  being  sharply  questioned.  One  of  the  most 
frequent  objections  to  direct  lighting  is  the  glare  which  is  caused 
by  the  exposed  light.  One  authority  states : 

What  constitutes  scientific  and  practical  illumination  ? 
There  are  so  many  varieties  of  industries  and  the  conditions 
of  production  are  so  diverse  that  it  is  at  once  apparent  that 

a) 


no  one  specification  can  fit  them  all.  Illumination  entirely 
satis i'&(fto"ry  under  on^.  set  of  conditions  might  be  wholly 
inadequate,  not  to  say  impossible,  in  another.  There  are, 
however,  some  general  principles  involved  which  apply  to 
practically  all  situations. 

There  is  a  physiology  as  well  as  a  psychology  of  indus- 
trial lighting.  The  psychological  states — the  mental  alert- 
ness and  the  attitude  toward  the  work — are  mostly  reflexes 
of  the  physiological.  It  is  worth  while  to  consider  how 
these  come  about,  and  how  those  that  tend  towards  ineffi- 
ciency may  be  eliminated. 

The  eye  is  a  most  delicate  mechanism,  just  as  delicate 
in  the  roughest  laborer  as  in  the  trained  artist.  It  is  a 
living  photographic  camera.  It  has  every  essential  feature 
of  the  camera  and  some  besides.  The  iris,  for  example, 
which  is  a  diaphragm  regulating  the  amount  of  light  ad- 
mitted to  the  eye,  is  wholly  automatic,  adjusts  itself  to 
meet  varying  conditions  of  illumination;  the  lens  which 
focuses  the  light  upon  the  retina  (the  sensitized  plate  in 
the  camera)  is  also  automatic,  and  adjusts  itself  for  dis- 
tance. The  retina,  under  stimulus  of  light  rays,  becomes 
fatigued ;  the  lens  and  iris  are  controlled  by  minute  muscles 
which  also  tire  under  constant  use.  Abuse  of  these  deli- 
cate mechanisms  leads  to  impotency,  just  as  overuse  of  any 
other  organ  does — a  set  of  muscles,  say,  which,  when  tired, 
cannot  be  made  to  work  so  strongly,  as  continuously,  or 
as  accurately  as  when  normal. 

Everybody  is  aware  of  the  effect  of  gazing,  even  for 
a  short  time,  at  any  bright  light.  The  eye  becomes  dazzled, 
irritated ;  if  continued  there  is  even  pain.  The  retina 
becomes  fatigued  by  overstimulation.  Also  the  muscles 
of  the  iris,  and  lens,  in  their  effort  to  adjust  themselves, 
are  tired.  The  effect  of  this  does  not  wear  off  immedi- 
ately. The  blinding  effect  persists  to  some  degree  for  a 
long  time,  often  a  half  hour  or  more,  according  to  the 
length  of  exposure  and  the  intrinsic  brightness  of  the  light. 

The  tense  feeling,  the  strain,  and  often  the  tingling 
and  itching  of  the  eye  following  continued  close  applica- 
tion, as  in  reading  fine  print,  or  closely  observing  fine 
work,  is  also  well  known.  It  is  in  part,  at  least,  the  direct 
and  indirect  result  of  a  fatigue  of  the  ciliary  muscles 
controlling  the  shape  of  the  crystalline  lens.  When  look- 
ing at  objects  near  at  hand  the  muscles  tighten  and  make 
the  lens  more  convex.  For  distances  of  fifteen  feet  and 

(2) 


beyond,  in  normal  eyes  the  image  focuses  on  the  retina 
when  the  ciliaries  are  relaxed.  If  the  eye  is  allowed  to 
relax  occasionally  by  a  look  at  relatively  distant  objects, 
fatigue  is  to  a  considerable  extent  avoided.  Likewise  the 
iris,  the  disklike  curtain  which  controls  the  size  of  tile 
pupil  and  therefore  the  amount  of  light  admitted,  if  kept 
tense  by  an  exposure  to  high  light,  or  if  subjected  to  fluc- 
tuating intensities,  also  tires  and  the  sense  of  discomfort 
is  increased.  Moreover  it  is  a  peculiarity  of  the  sense  of 
light  that  movement  or  change  of  any  sort  taking  place 
outside  the  limited  field  of  direct  vision,  but  still  within  the 
range  of  perception,  is  percieved  more  quickly  even  than 
when  in  direct  range;  and  the  effort  of  the  eye  to  adjust 
itself  to  these  environmental  conditions  when  the  light  is 
unsteady  or  the  surroundings  dark  in  comparison  with  the 
field  upon  which  attention  is  concentrated,  is  not  only 
annoying  and  fatiguing,  but  becomes  intensely  distracting 
to  anyone  compelled  to  face  it. 

Here  then  we  have  a  combination  of  paraial  visual 
paralysis,  optic  fatigue,  general  discomfort  and  distraction 
from  the  work  in  hand,  with  all  the  physical  and  mental  ills 
and  the  inefficiencies  following  in  their  train.  Evidently 
the  physiological  effects  of  lighting  are  very  practical  fac- 
tors in  illumination,  quite  as  practical  as  the  immediate 
dollars-and-cents  considerations  which  are  more  easily 
sensed. 

It  must  be  apparent  that  scientific  and  therefore  eco- 
nomic lighting  not  only  provides  a  sufficiency  of  light  flux, 
steady  and  so  distributed  as  to  avoid  obscurity  and  deep 
shadows  anywhere  within  the  ordinary  range  of  the 
workers'  eyes,  but  takes  care  that  the  source  of  illumination, 
if  considerably  concentrated  as  in  the  case  in  most  lighting 
units  now  in  use,  shall  be  concealed,  enclosed  in  a  diffusing 
envelope,  or  placed  outside  the  range  of  vision  .unless  at 
such  a  distance  that  the  brilliancy  is  greatly  reduced  before 
the  rays  reach  the  eye.  Otherwise  the  main  purpose  of 
lighting  is  defeated,  and  the  workers  themselves  are  physi- 
cally harmed  and  are  rendered  less  efficient  for  their  work. 

Common  practice  is  to  have  individual  desk  lamps.  These 
throw  a 'very  strong  light  upon  the  work  and  nowhere  else. 
In  spite  of  the  fact  that  most  clerks  working  with  a  lamp 
of  this  sort  wear  dark  green  eye  shades,  the  strain  upon  the  eyes 
is  very  great.  In  still  other  places  there  are  individual  lamps 

(3) 


as  well  as  overhead  illumination.  This  arrangement  gives  bet- 
ter light  than  the  individual  lamps  but  it  is  not  economical. 
If  direct  lighting  be  used  it  is  much  better  to  have  uniform 
illumination  at  least  eight  or  ten  feet  above  the  floor,  using  as 
many  lamps  as  will  give  each  desk  enough  light  for  comfortable 
work.  The  light  coming  from  so  many  directions  reduces  the 
glare  more  or  less  and  heavy  shadows  are  not  so  noticeable.  It 
is  helpful  if  the  light  units  be  placed  so  that  the  upright  posts 
do  not  shut  off  she  light.  The  diagram  on  this  page  shows 
Mr.  Becker's  plan  for  the  proper  arrangement  of  direct  lights. 


Extent  of  Effective  Reach  of 


HOW  TO  SPACE  LAMPS 

The  diagram  shown  here  gives  a  good  idea  of  one  of  the  ways  that  light  may  be 
evenly  distributed.     Many  offices  have  an  uneven  distribution  of  the  light  and 
in  such  cases  desk  lamps  are  an  expensive  necessity.    With  the  spacing  indi- 
cated in  the  diagram,  satisfactory  results  are  usually  obtained 

If  you  use  the  60  or  100  watt  tungsten  lights  you  will  probably 
get  the  best  results  simply  because  the  light  source  is  not  brilliant 
enough  to  cause  excessive  glare.  Ordinarily  the  unprotected 
nitrogen  or  gas-filled  lamp  of  high  intensity,  although  the  eco- 
nomy in  current  consumption  is  more  apparent,  should  never 
be  used  for  the  direct  lighting  of  offices,  since  the  glare  from 
this  light  is  often  injurious  to  the  eyes. 

The  term  semidirect,  or  semiindirect  lighting,  is  applied  to 
the  method  of  casting  light  on  the  walls  or  ceiling  by  means  of 
translucent  reflectors,  which  is  the  only  difference  between 

(4) 


indirect  and  semiindirect  system  of  lighting.  The  current 
consumption  for  an  equal  amount  of  semiindirect  light  is  said 
to  be  from  forty  to  fifty  per  cent  more  than  for  a  direct  light- 
ing system  properly  designed,  while  the  current  consumption 
for  a  purely  indirect  system  is  stated  to  be  from  fifty  to  seventy- 
five  per  cent  more  than  the  direct.  This  difference  probably 
explains  the  frequent  compromises  resulting  in  the  semiindirect 
systems.  Undoubtedly  the  semiindirect  system  of  lighting  is 
less  tiresome  to  the  eyes  than  the  direct  system,  but  some  office 
managers  question  whether  there  is  any  real  saving  over  the 
indirect  systems. 

With  indirect  lighting  the  eyes  are  protected  from  the  bril- 
liancy of  the  lamps,  since  the  light  reflected  from  the  ceiling 
is  largely  diffused  before  reaching  the  working  plane.  Indirect 
lighting  is  perhaps  the  only  practical  means  of  hiding  the  lamp 
without  cutting  down  its  light  giving  qualities.  Where  this 
system  is  used  the  ceiling  or  reflecting  surface  must  of  course 
be  white.  When  offices  are  located  in  factories  or  buildings  of 
mill  construction  this  is  often  impossible,  but  usually  even  there 
it  is  possible  to  get  a  fairly  good  reflecting  surface  by  painting 
the  ceiling  properly. 

From  the  standpoint  of  eye  efficiency,  indirect  lighting  is 
about  the  nearest  approach  to  sunlight  that  can  be  devised.  It 
is  true  that  the  consumption  of  current  by  this  method  is  from 
forty  to  seventy  per  cent  greater,  but  some  managers  who  are 
using  this  system  believe  that  the  increased  efficiency  of  the 
workers  p&ys  for  the  difference  in  cost  many  times  over. 

Dr.  C.  E.  Ferree  has  arrived  at  some  very  interesting  con- 
clusions. Some  of  them  have  been  disputed  by  electrical  illum- 
inating engineers,  but  his  experiments  were  very  thorough  and 
the  results  should  cause  a  widespread  investigation. 

Dr.  Ferree  started  out  to  find  whether  or  not  the  eye  shows 
a  loss  of  efficiency  after  three  or  four  hours'  work  under 
a  given  lighting  system.  He  prepared  a  series  of  test  cards 
from  which  the  factor  of  memory  was  eliminated,  containing 
a  number  of  conventional  letters  and  lines  at  which  the  subject 
was  required  to  look  intently  and  record  on  a  rotating  drum  the 
time  during  which  he  saw  them  clearly  and  the  time  he  saw  them 
blurred.  This  test  was  continued  during  periods  of  three  and 

(5) 


four  hours  and  a  ratio  established  showing  the  proportion  of 
clear  vision  time  to  blurred  vision  time.  Those  who  wish  to 
go  further  into  the  detail  of  Dr.  Ferree's  experiments  will  find 
his  paper  unusually  clear  and  interesting.*  Dr.  Ferree  states: 

The  intensity  recommended  by  the  Illuminating  Engi- 
neering Society  in  its  primer  issued  in  1912  ranges  from 
two  or  three  to  seven  or  ten  foot  candles,  depending  upon 
the  kind  of  work.  Five  foot  candles  are  taken  as  a  medium 
value.  This  medium  value  is  approximately  three  times 
the  amount  we  have  found  to  give  the  least  loss  of  efficiency 
for  the  type  and  installation  of  semi  direct  lighting  we 
have  used.  The  intensity  we  have  found  to  give  the  least 
loss  of  efficiency  for  this  type  of  lighting  does  not  give 
maximum  acuity  of  vision  as  determined  by  the  momentary 
judgment.  At  an  intensity  that  does  give  maximum 
acuity  of  vision  as  determined  by  the  momentary  judg- 
ment the  eye  runs  down  rapidly  in  efficiency.  That  is,  in  this 
type  of  lighting  one  or  the  other  of  these  features  must  be 
sacrificed.  High  acuity  and  little  loss  of  efficiency  cannot  be 
had  at  the  same  intensity.  This  can  be  had  only  under 
daylight  or  under  the  indirect  systems  of  artificial  lighting. 
However,  the  amount  of  light  we  find  to  give  the  least  loss 
of  efficiency  seems  to  be  sufficient  for  much  of  the  work 
ordinarily  done  in  the  office  or  home. 

The  semiindirect  systems  of  lighting  are  intended  to 
represent  a  compromise  between  the  direct  and  indirect 
systems.  A  part  of  the  light  is  transmitted  directly  to  the 
eye  through  the  translucent  reflectors  placed  beneath,  and 
a  part  is  reflected  to  the  ceiling.  By  giving  better  distri- 
bution, this  system  is  also  supposed  to  be  a  concession  to 
the  welfare  of  the  eye,  but  our  tests  show  that  the  conces- 
sion is  not  so  great  as  it  was  supposed  to  be.  In  fact,  in- 
stalled at  the  intensity  ordinarily  used,  or  at  an  intensity 
great  enough  for  all  kinds  of  work,  it  is  little  better  for 
the  eye  than  the  direct  system.  At  these  intensities  the 
bright  sources  of  light,  which  seem  to  be  the  fundamental 
cause  of  damage,  have  not  been  eliminated,  nor  even  re- 
duced, so  as  to  give  much  relief  to  the  suffering  eye.  Until 
this  is  done  in  home,  office  and  public  lighting  we  cannot 
hope  to  get  rid  of  eye  strain  with  its  complex  train  of 
physical  and  mental  disturbances. 


*"The  Efficiency  of  the  Eye  under  Different  Systems  of  Illumination,"  by  C.  E. 
Ferree,  a  paper  read  at  the  seventh  annual  convention  of  the  Illuminating  Engineering 
Society,  Pittsburgh,  Pa.,  Sept.  22-26,  1913. 

(6) 


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